This invention relates to improvements in an exhaust gas purifying catalyst and a method of producing the same, and more particularly to the exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine so as to remove hydrocarbons (IC), carbon monoxide (CO) and nitrogen oxides (NOx) contained in exhaust gas, particularly effectively removing hydrocarbons. The exhaust gas purifying catalyst is effective, for example, for purifying exhaust gas discharged from the engine of an automotive vehicle or the like in a cold engine operation range (where exhaust gas temperature is low) immediately after engine starting, particularly removing hydrocarbons at a high efficiency.
In recent years, a so-called HC adsorbing catalyst has been developed for the purpose of removing a large amount of hydrocarbons generated in a low engine temperature range during engine starting of an internal combustion engine. The HC adsorbing catalyst includes a HC adsorbing material layer containing zeolite as a main component, and a three-way catalyst component layer which are coated over a monolithic substrate. With such a HC adsorbing catalyst, the HC adsorbing material layer is adapted to temporarily adsorb and retain a large amount of HC generated in the low exhaust gas temperature range or during engine starting in which the three-way catalyst cannot effectively function. Thereafter, the adsorbed and retained HC is gradually released or desorbed from the HC adsorbing material layer and oxidized to be removed when exhaust gas temperature rises to activate the three-way catalyst component.
Now, in the above HC adsorbing catalyst, there is an interrelation between kind of hydrocarbons (HC) in exhaust gas and pore diameters of the zeolite, and therefore it is required to use the zeolite having suitable pore diameters for the kind of hydrocarbons to be adsorbed. In view of this, MFI (zeolite) as a main component is blended with other zeolite having different pore diameters (for example, USY-type zeolite) so as to obtain a HC adsorbing catalyst component whose pore diameter distribution is adjusted. However, since stability of the crystal structure and degree of deformation of pores are different depending upon kind of zeolites under temperatures and in the presence of water vapor, difference in adsorption and desorption characteristics is enlarged among kinds of the zeolites after a long time use (or durability test) so that adsorption and retention ability for hydrocarbons in exhaust gas may become insufficient.
In view of the above, in order to ensure an adsorbing performance for hydrocarbons, using a heat-resistant zeolite having a high silica-alumina ratio (Si/2Al ratio) has been proposed. Such zeolite is called a xe2x80x9chigh-silca zeolitexe2x80x9d.
Concerning the three-way catalyst component, it has been proposed that noble metals such as rhodium (Rh), platinum (Pt) and palladium (Pd) coexist in the same layer coated on a substrate, or that Ph and Pd exist respectively in separate layers coated over a substrate. In this regard, Japanese Patent Provisional Publication No. 2-56247 has proposed an exhaust gas purifying catalyst in which a second layer containing the noble metals such as Rh, Pt and Pd as a main component is formed on a first layer containing zeolite as a main component.
Additionally, in order to-lower the concentration of HC in exhaust gas discharged in a low engine temperature condition made immediately after engine starting, it has been proposed that HC is temporarily stored in a HC adsorbing material and released after a three-way catalyst disposed downstream of the HC adsorbing material has been activated thereby to oxidize the released HC. Such proposition is disclosed in Japanese Patent Provisional Publication Nos. 6-74019, 7-144119, 6-142457, 5-59942, and 7-102957.
However, with the above conventional HC adsorbing catalysts, when the crystal structure of zeolite is broken and deformation of pores in zeolite is promoted in the HC adsorbing material layer after a long time use, the absorbed HC is unavoidably released from zeolite before the three-way catalyst components in the three-way catalyst component layer as an upper layer is activated. Consequently, the oxidizing efficiency for the released HC is sharply lowered so that the HC adsorbing catalyst cannot function as an exhaust gas purifying catalyst.
Besides, it is effective to increase the Si/2Al ratio of zeolite and introduce polyvalent metal into the zeolite frame in order to improve heat resistance of zeolite. However, use of a high silica zeolite increases a hydrophobic property of surface of zeolite particles thereby largely lowering a bonding ability of zeolite particles with an inorganic binder. Accordingly, zeolite unavoidably separates and settles in an aqueous slurry thereby degrading a coating characteristics of the slurry. Additionally, after a long time use, the bonding force between the HC adsorbing material layer and the three-way catalyst component layer is sharply lowered so that the three-way catalyst component layer may peels off from and separates from the substrate. As a result, it is impossible to equally effectively remove all of HC, CO and NOx.
Furthermore, with exhaust gas purifying systems including the HC adsorbing catalyst disclosed in the above Japanese Provisional Publications, the durability of the HC adsorbing material is insufficient, and therefore a HC adsorbing ability of the HC adsorbing material is lowered after a long time use. Additionally, the HC absorbed in the upstream stage HC adsorbing material is released before the downstream stage three-way catalyst has been activated, thereby degrading an exhaust emission condition. In this regard, in order to improve the adsorbing ability of the HC adsorbing material and to accomplish retardation of HC releasing, it has been proposed to cause high temperature exhaust gas to flow through a bypass passage provided with the HC adsorbing material, or to preheat exhaust gas containing released HC by means of a heat exchanger prior to introduction of exhaust gas to the three-way catalyst.
However, these measures make the exhaust gas purifying systems complicated while sharply increasing production cost of the systems although sufficient effects for equally removing HC, CO and NOx cannot be obtained. Thus, development of the HC adsorbing material which is high in durability and HC adsorbing ability has been eagerly desired.
It is an object of the present invention to provide an improved exhaust gas purifying catalyst and a method of producing the same catalyst, which can effectively overcome drawbacks encountered in conventional exhaust gas purifying catalysts.
Another object of the present invention is to provide an improved exhaust gas purifying catalyst and a method of producing the same catalyst, which can effectively purify exhaust gas throughout a long period of time.
Another object of the present invention is to provide an improved exhaust gas purifying catalyst and a method of producing the same catalyst, which can effectively oxidize or remove a high concentration of HC emitted in a cold engine operation range or immediately after engine starting while effectively preventing a HC adsorbing material (HC adsorbing catalyst component) from peeling off and separating from a substrate even after a long time use of the exhaust gas purifying catalyst.
An aspect of the present invention resides in an exhaust gas purifying catalyst which comprises a substrate. A hydrocarbons adsorbing material layer for adsorbing hydrocarbons contained in exhaust gas is formed as a lower layer over the substrate and contains zeolite and colloidal silica which has undergone calcining. The colloidal silica is in a chain structure and/or a spherical structure before and after the calcining. Additionally, a catalyst component layer is formed as an upper layer over the hydrocarbons adsorbing material layer. The catalyst component layer contains a catalyst metal.
Another aspect of the present invention resides in an exhaust gas purifying catalyst which comprises a substrate. A hydrocarbons adsorbing material layer for adsorbing hydrocarbons contained in exhaust gas is formed as a lower layer over the substrate and contains zeolite and silica particle. The silica particle adheres to surface of zeolite and is in a chain structure and/or a spherical structure. Additionally, a catalyst component layer is formed as an upper layer over the hydrocarbons adsorbing material layer. The catalyst component layer contains a catalyst metal.
A further aspect of the present invention resides in an exhaust gas purifying catalyst which comprises a substrate. A hydrocarbons adsorbing material layer for adsorbing hydrocarbons contained in exhaust gas is formed as a lower layer over the substrate and contains zeolite and silica particle. The hydrocarbons adsorbing material layer is formed by coating a coating liquid containing zeolite and colloidal silica which is in a chain structure and/or a spherical structure, over the substrate. Additionally, a catalyst component layer is formed as an upper layer over the hydrocarbons adsorbing material layer. The catalyst component layer contains a catalyst metal.
A still further aspect of the present invention resides in an exhaust gas purifying catalyst which comprises a substrate. A hydrocarbons adsorbing material layer for adsorbing hydrocarbons contained in exhaust gas is formed as a lower layer over the substrate and containing zeolite and colloidal silica which has undergone calcining. The zeolite has undergone a treatment for providing hydrophobic property with mesoporous silica. The colloidal silica is a chain structure and/or a spherical structure before and after the calcining. The colloidal silica has an average particle size ranging from 5 to 100 nm before the calcining. The colloidal silica is contained in an amount ranging from 1 to 50% by weight relative to the hydrocarbons adsorbing material layer after the calcining. Additionally, a catalyst component layer is formed as an upper layer over the hydrocarbons adsorbing material layer. The catalyst component layer contains a catalyst metal.
A still further aspect of the present invention resides in a method of producing an exhaust gas purifying catalyst. The method comprises (a) coating a first coating liquid over a substrate to form a hydrocarbons adsorbing material layer as a lower layer, the first coating liquid containing zeolite and colloidal silica which is in a chain structure and/or a spherical structure; and (b) coating a second coating liquid on the hydrocarbons adsorbing material layer to form a catalyst component layer as an upper layer, the second coating liquid containing a catalyst metal.